I built the Edge Barracuda over a period of about 4 1/2 years. It was my first build and it was a hell of an experience. I learned a ton!
You have to go into it knowing that it will take up a lot of your time. I took longer than most due to expense more than not having time to work on it. There were times when I didn't even want to look at it so I'd step away for a few months and save money for parts and then hit it hard again.
If you aren't a machinist then it will get expensive to buy parts that you cannot make on your own. I recommend finding build threads of any buggy you are interested in and study them so you know what you are getting into.
I figured I could build it for $8-9K. I got laughed at. $14K later and now I know why people were laughing.
I bought my 2004 GSX-R1000 on ebay for $1100. If you buy the motor without the bike make sure you get the harness, ECM, and airbox. The header may or may not be a bonus. The 2004 came with the header, but it was titanium and I couldn't weld it anyway so I fabricated my own.
I would also look for mini projects that people have given up on. There are people that start and realize they bit off more than they could chew and want to walk away so you might score some components for a deal.